1. Field of the Invention
The present invention relates to a semiconductor device in which a leak current from a metal-oxide-semiconductor (MOS) capacitor to a silicon substrate is suppressed.
2. Description of the Related Art
When a MOS capacitor is formed on a silicon substrate and used at the same voltage as that applied to the silicon substrate, an electrode for the silicon substrate side of the MOS capacitor must be formed on a well region of an opposite conductivity to the silicon substrate. When the MOS capacitor has a large area and is used under a high temperature, in particular, the leak current becomes considerably large between the silicon substrate and the well region in which the electrode for the silicon substrate side is formed, causing a problem in circuit construction.
Conventional ways of avoiding the above-mentioned leak current include a method involving forming a capacitor with a first polysilicon layer and a second polysilicon layer being opposing electrodes, and a method involving separating, as in silicon on insulator (SOI), the silicon substrate and the well region in which the electrode on the silicon substrate side is formed, by an oxide film.
Apart from the problem of the leak current, as a way of realizing high integration of capacitors in a dynamic random access memory (DRAM) cell, conventionally, a trench capacitor which is formed by utilizing a recess surface of a trench formed in a silicon substrate has been used as described in JP 02-165663 A.
As has been described above, as a way of suppressing the leak current parasitically flowing from the capacitor to another circuit, when the capacitor is formed of two polysilicon layers, there is a need to add a step of forming the second polysilicon layer, and because the leak current between the electrodes is large compared with a capacitor formed of a silicon substrate and polysilicon interposing a gate oxide film therebetween, quality of an insulating film between the first polysilicon layer and the second polysilicon layer needs to be optimized. Further, when the well and the silicon substrate are separated by the oxide film using a SOI substrate, a cost for the substrate increases, becoming a problem.